This proposal describes a 5 year Mentored Clinical Investigator Career Development Award in Muscle Diseases Research as detailed in the NIAMS mission statement. The MD/PhD principal investigator is a board-certified pediatric neurologist. He will now expand upon his scientific skills through a unique integration of interdepartmental resources and establish a scientific niche to construct an academic career. This program will promote the longstanding interest of the PI in Duchenne Muscular Dystrophy (DMD) by expanding into molecular and developmental biology and mouse molecular genetics. Dr. Lee Niswander will mentor the principal investigators scientific development. A Howard Hughes Medical Investigator, Dr. Niswander is a recognized leader in the field of developmental biology and mouse genetics and has trained numerous postdoctoral fellows, graduate students and clinical fellows. In addition, an advisory committee of highly-regarded muscle scientists will provide scientific, clinical and career advice to help the PI develop into an independent investigator. The PI is in an ideal setting to incorporate expertise from diverse resources and utilize multidisciplinary technologies. Research will focuses on the hypothesis of de novo creatine synthesis in dystrophin-deficient mdx mice. The PI has identified guanidineacetate methyltransferase (GAMT), the key enzyme for creatine synthesis as being differentially upregulated at both the mRNA and protein level in mdx skeletal muscle. Using an assortment of biochemical, molecular, cellular and magnetic resonance imaging techniques, the specific aims include: 1) As GAMT is upregulated in mdx mice, the end-product creatine should also be increased. However because of membrane leakage, increased creatine levels may be largely found in urine and serum, not muscle, 2) Determine if AGAT (the precursor enzyme with GAMT required for creatine synthesis), is also upregulated at the mRNA and protein level in mdx muscle, and 3) Determine the phenotype of a double-null GAMTimdx mouse. This will be the first detailed analysis of creatine biosynthesis in the mdx mouse. Objective is to determine if the mdx mouse synthesizes creatine de-novo as an adaptation to mitigate metabolic compromise caused by leaky membranes. Dystrophin is an important protein for normal muscle function. Boys with DMD do not make dystrophin and their muscles weaken over time. The mdx mice do not make dystrophin yet they do not become crippled. These studies may offer important clues that could help with new treatments for boys with DMD.